You're only treating the trial worse if your guild is bad and not worth getting invited to. Otherwise the trial that gets invited to be a regular raid member is getting a far better deal than the pug who gets brought along for a couple of bosses. Joining as a regular member means getting to down bosses on a regular basis, more chance at getting more loot in the future (and getting priority on the specific pieces you really want), and the chance to work on progression bosses rather than just be brought along to farm loot (which I'd imagine is what interests a lot of serious raiders more).
To use your previous analogy, its like the difference between a temp that comes in and does the job for a couple of days and is then gone, and the person who gets hired as a full time employee with full benefits, opportunities for promotion and future stability. Theres a lot of benefits to being part of a solid raiding guild than just "How much loot can I get".